LATE QUATERNARY CRUSTAL UPLIFT AND SOUTHWARD SHIFT OF THE LHASA RIVER IN THE LHASA AREA, TIBET

On the basis of the field investigation, digital elevation modeling and remote sense data analysis, the authors think that the nearly E-W-trending watersheds and valleys in the Lhasa area are distributed alternately from north to south under the nearly N-S compression, parallelly to the WNWtrending main faults.From the erosional surfaces of the summits near the watersheds to catchment basins, the hill slope changes from being gentle to being relatively steep and again to being gentle, indicating a successive change from the hilltop erosion zone through intense erosion zone to sedimentation zone and a complete process of erosion and sedimentation under the tectonic-climatic control.Under the same climatic conditions, the Lhasa block tilted to the south during the late Quaternary neotectonic movement, and as a result, the northern Lhasa block has been eroded more intensely than the southern block, and therefore the downstream reaches carry more sand and mud.As more alluvial-diluvial fan deposits from the northern block flow to the Lhasa River than those from the southern block and in addition the crust tilts from north to south, the Lhasa reach of the Lhasa River has shifted toward the south and so has the alluvial fan of the Doilung Qu river mouth.